1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing and more specifically to a method and apparatus for mixing a precise amount of chemicals in a single wafer process.
2. Discussion of Related Art
Wet etching and wet cleaning of silicon wafers is typically done by immersing the wafers into a liquid. This can also be done by spraying a liquid onto a wafer or a batch of wafers. Wet wafer cleaning and etching is traditionally done in a batch mode. Because of the need for a shorter cycle time in chip manufacturing, there is a need for fast single wafer processing. When using single wafer processing, the amount of chemicals in processes is much smaller than when using batch processing. Even though the quantities in use at any time are much smaller than in batch processing, the accuracy of mixing has to be similar to batch processing.
When performing wet etching operations, the accuracy of the etch has to be smaller than 1% 1 sigma total variation on a 300 mm wafer. This variation is the result of variations in contact time over the wafer when spraying chemicals, the variation in temperature in the etching chemical and on the wafer surface and the variation in chemical concentration. Therefore the variation in chemical concentration has to be controlled very tight. When using wet chemicals for cleaning instead of etching wafers, the accuracy of mixing can be relaxed. Traditionally, in batch equipment, the chemicals are premixed in an off-line tank, where water and chemicals are added separately. Usually, at first chemicals are added and the amount is monitored by monitoring the level. Then the water is added to the full level. The chemicals in this off-line tank can be heated and when needed are transferred to the etching or cleaning tank. Inside the tank the concentration can be monitored and additional chemical or water can be added to adjust for any variations. Alternatively, such as in a flow-through reactor (e.g. CFM Technologies), chemicals are measured in a tube in which the level is monitored and are injected in a stream of DI water of which the flow is controlled. These techniques work well for mixing chemical volumes of the order of 1 to 41 of chemicals with multiple volumes of DI water.
Most single wafer wet processors available today use a similar principle. I.e., chemicals are premixed in an off-line tank and then are pumped to the single wafer chamber when needed. The problem with this approach is that for every mixing ratio of chemical, a specific mixing tank has to be constructed and chemicals have to be mixed in quantities far exceeding the necessary amount for the processing of one wafer.
Thus, there is a need for a simple and accurate mixing system coupled to a single wafer wet processing chamber that can be connected to the bulk supply of the semiconductor fab directly, without the use of a big pre-mixing tank for multiple wafer processing